Updated constraints on amplitude and tilt of the tensor primordial spectrum

TL;DR

This paper updates the constraints on the primordial tensor spectrum's amplitude and tilt using multiple datasets, establishing the most stringent limits to date and analyzing the compatibility of NANOGrav signals with inflationary gravitational waves.

Contribution

It provides the strongest current constraints on the tensor-to-scalar ratio and spectral index, and compares methods for analyzing the tensor sector in cosmological data.

Findings

Tensor-to-scalar ratio constrained to r<0.028

Tensor spectral index range: -1.37 to 0.42

NANOGrav signal incompatible with simple primordial inflation models

Abstract

We have taken a comprehensive approach to update the limits on the tensor-to-scalar ratio ($r$) and the tensor spectral index ($n_t$), using 10 datasets from the BICEP/Keck Array 2015 and 2018, Planck releases 3 and 4, and LIGO-Virgo-KAGRA Collaboration. By fitting the complete $\Lambda$CDM+$r$+$n_t$ model with two different approaches for the tensor sector, we have not only established which method is the most reliable, but have also achieved the strongest constraint on the tensor-to-scalar ratio in current literature: $r<0.028$ and $-1.37 < n_t < 0.42$ at 95% confidence level. Furthermore, our examination of the common signal detected by the NANOGrav Collaboration further confirms that a simple power-law cannot reconcile the constraints from different datasets if the NANOGrav detection is due to a primordial inflationary gravitational wave background, as previously shown in the literature.